Sleeper Simulant, hands down. I soloed the entire Gjallarhorn quest on all of my characters, and while fun and interesting, there was never a point where I was frustrated enough to ask for help (not even close). The last mission of the Sleeper Simulant quest (the special version of the Sabre II strike), I was severely under leveled for, but luckily I was match made with a bad ass and we eventually made it through as a 2 man team. I probably could have soloed it if I were properly leveled at the time, but I wasn't and (clearly) couldn't.